1. Field of the Invention
The subject matter of this invention pertains to a process for preparing molded polyurethane-polyurea elastomers by reacting an organic polyisocyanate with a reactive polyol composition which contains certain diaminodiphenylmethane compounds as chain extenders.
2. Description of the Prior Art
It is known that polyurethane compositions can be prepared by reacting an organic polyisocyanate with a compound containing at least two active hydrogen atoms as determined by the well known Zerewitinoff method. However, one of the challenges facing those who work in the polyurethane field is to prepare molded polyurethane parts. It has been especially challenging to prepare large molded parts for automobiles such as facia, bumpers, hoods, doors, and fenders. Such parts must have good mechanical properties. In particular, the changes in modulus of elasticity with change in temperature should be minimized. The parts must be sufficiently rigid at high temperatures and sufficiently flexible at low temperatures.
Not only must the parts have good mechanical properties, but the ingredients used in preparing the polyurethane parts must lend themselves to reaction injection molding (RIM) processing. The ingredients must not plug up the feed lines or the mix heads, and must allow quick injection times. The mold flow characteristics and the reactivity of the ingredients must be such that the mold cavities are completely filled so that the parts will be completely shaped. On the other hand, demold times must be minimized to allow faster cycle times.
One of the primary methods of making polyurethane products with improved mechanical properties, is to add a chain extender, such as a low molecular weight diol, to the polyurethane formulation. Although chain extenders such as 1,4-butanediol will provide polyurethane products with improved mechanical properties, the demolding time can be as long as 1 to 4 minutes. Also, the compositions do not possess adequate modulus at higher temperatures.
Because of the limitations of using low molecular weight diols such as 1,4-butanediol as chain extenders, aromatic amines have been used as chain extenders in polyurethane formulations. These have been used because the reaction of aromatic amines with organic polyisocyanates will result in polymers with an increased proportion of urea linkages. The urea linkages improve the mechanical properties of the products, especially their resistance to higher temperatures.
U.S. Pat. No. 3,428,610, for instance, discloses the use of aromatic diamines as chain extenders. The aromatic diamines used have (a) one linear alkyl substituent of 1 carbon atom to 3 carbon atoms in an ortho position to one amino group and two linear alkyl substituents of 1 carbon atom to 3 carbon atoms in both ortho positions to the other amino group, or (b) two linear alkyl substituents of 1 carbon atom to 3 carbon atoms in both ortho positions of each of the amino groups. British Pat. No. 1,408,943 broadly discloses the use of diaminodiphenylmethane and polyaminopolyphenylmethane compounds as chain extenders. Among the dozens of compounds specifically mentioned is 3,3',5,5'-tetraisopropyl-4,4'-diaminodiphenylmethane.
However, both U.S. Pat. No. 3,428,610 and British Pat. No. 1,408,943 are restricted to a two step prepolymer process in which an isocyanate is reacted with a polyol in a first step, and in the second step the resulting prepolymer is reacted with the diamine to form a polyurethane-polyurea elastomer. The British Patent discloses that the reason a prepolymer method is used is because it is important that the reactivity of the polyisocyanates and polyamines be adjusted to a similar level. The polyisocyanates are made less reactive by reacting them with the polyol before the reaction with the diamine. See British Pat. No. 1,408,943, page 1, lines 28-32.
British Pat. No. 1,534,258 on the other hand, discloses the use of certain diamines as chain extenders for the production of polyurethane-polyurea elastomers by the reaction injection molding (RIM) technique. This patent indicates that prepolymers are not required to reduce the reactivity of the polyisocyanates when a RIM process is used. The diamines included as chain extenders in the patent are any active aromatic diamines or higher polyamines. An active diamine is defined as one whose reactivity towards isocyanates has not been reduced by electron attracting substituents. According to the patent, this definition includes unsubstituted compounds such as 2,4'- and/or 4,4'-diaminodiphenylmethane and methyl substituted compounds such as 2,4- and 2,6-diaminotoluene.
The patent further states that aromatic amines which have proved to be particularly suitable as chain extenders, contain at least one linear alkyl substituent in the ortho-position to the first amino group and two linear alkyl substituents having from 1 to 3 carbon atoms in the ortho positions to the second amino group. Examples of such compounds include 1,3-diethyl-2,4-diaminobenzene, 2,4-diaminomesitylene, 1-methyl-3,5-diethyl-2,4-diaminobenzene, 1-methyl-3,5-diethyl-2,6-diaminobenzene, 1,3,5-triethyl-2,6-diaminobenzene, and 3,3',5,5'-tetraethyl-4,4'-diaminodiphenylmethane.
Furthermore, the patent says that it is preferred to use aromatic diamines which have a linear alkyl substituent having from 1 to 3 carbon atoms and in both orthopositions to each amino group. It states that for processing by the RIM process, it is advantageous that one of these diamines is liquid at room temperature and miscible with polyhydroxyl compounds in any proportion. In this regard, 1-methyl-3,5-diethyl-2,4-diaminobenzene or a mixture of this compound with 1-methyl-3,5-diethyl-2,6-diaminobenzene is the preferred compound to use as a chain extender.
In contrast to the teachings of British Pat. No. 1,534,258, the applicants have found that certain aromatic diamines with non linear alkyl substituents in the ortho position to the amino groups provide unexpected advantages over aromatic diamines with linear substituents as chain extenders in the preparation of molded polyurethane-polyurea parts by RIM processes.